The present invention relates to a process for preparing commercial quality halogenated ethanes, and particularly pentafluoroethane from a mixture produced by the reaction of perchloroethylene and hydrogen fluoride. In particular, this invention provides a process that minimizes the hydrogen fluoride that is carried off with the desired products after they are separated from the reaction mixture, and at the same time reduces the need to recycle undesirable byproducts to the reaction.
Pentafluoroethane (herein referred to as R125) is a hydrofluorocarbon (HFC), and chlorotetrafluoroethane (R124) and dichlorotrifluoroethane (R123) are hydrochlorofluorocarbons (HCFCs) that increasingly are being used to replace the environmentally disadvantageous chlorofluorocarbons (CFCs) in refrigeration and other applications. Furthermore, the HFCs and HCFCs are preferably as free as possible of CFCs. Some current regulations call for HFC and HCFC products to contain not more than 0.5 weight percent total CFCs as an impurity, and these regulations may become more restrictive in the future. It is therefore important that commercial HFC and HCFC products have a concentration of CFCs that is as low as possible.
The following table identifies the principal halogenated ethanes and other compounds which will be discussed in this application, and includes their refrigerant (R) numbers, formulas and boiling points at atmospheric pressure, with the compounds listed in boiling-point order. The data is from Stacey, et al., Advances in Fluorine Chemistry, pp. 173-175 (1963), except the boiling points for HF and HCl are from The Merck Index, Tenth Ed., 1983.
Unless indicated otherwise, hereinafter xe2x80x9cR124xe2x80x9d shall refer to R124, its isomer R124a, and mixtures of these. Similarly, xe2x80x9cR123xe2x80x9d refers to R123, its isomer R123a, and mixtures of these, while the term xe2x80x9cR114xe2x80x9d shall refer to R114, its isomer R114a, and mixtures of these. As can be seen, the difference in boiling points between each pair of isomers is relatively small.
The fluorination of PCE with HF is a well-known process used for the production of various fully and partially halogenated ethanes. See, for example, U.S. Pat. No. 3,755,477, incorporated herein by reference. In this process, the ethylene double bond of the PCE is broken, and hydrogen and fluorine from the HF attach to the two carbon atoms. As the process proceeds, chlorine atoms are successively replaced by fluorine, yielding HCl as a byproduct. At any given time the reactor contains a mix of unreacted PCE and HF, HCl, and various ethanes halogenated with different combinations of chlorine and fluorine atoms, depending on the balance of the different possible fluorination schemes. Various different fluorination progressions may occur simultaneously. For example, and with reference to the list of the halogenated ethanes set forth in Table I, in one fluorination sequence PCE becomes R1111, which successively converts to R121, R122, R123, R124 and finally R125, which is a desired end product. Alternatively, underfluorinated intermediates, including R123 and R123a, can convert to R113, then to R114, and finally R115. This latter reaction scheme is considered undesirable in the production of R125, because R115 is an environmentally undesirable chlorofluorocarbon which is difficult to separate from R125, as discussed below.
After the synthesis reaction, the desired end products generally are separated from the undesired byproducts, intermediates and unconverted starting materials. Conventional methods of separation often include distillation of the reactor product stream followed by water and/or caustic wash to remove acids and then a final drying. The present inventors have come to appreciate that such methods may result in significant losses of HF and may also generate waste streams that must be treated. The HF loss occurs because desired products, such as R125, R124, and R123, form azeotropes with HF. Therefore, with conventional distillation it is impossible to completely remove HF from the desired products. According to such prior processes, the stream or streams which contain the desired HFCs will carry with them slightly more than the azeotropic amounts of HF, thus necessitating further processing of the product stream to remove the HF by, for example, water absorption and/or caustic scrubbing.
Based on the above reaction schemes, the present inventors have appreciated that, in order to promote the production of R125, it is desirable to recycle the underfluorinated intermediates which are most readily fluorinated into R125, such as R121 and R122. Although R123 and R124 may be drawn off as desirable hydrochlorofluorocarbon end products, they may also be recycled to form additional R125. Some or all of the R123 and R124, which may be desirable end products, may also be recycled depending on the desired ratio of final products.
On the other hand, it is undesirable to recycle R114, which tends to fluorinate into R115 rather than R125. R115 is an undesirable CFC which is difficult to separate from R125. As discussed in U.S. Pat. No. 5,346,595, incorporated herein by reference, R125 tends to form an azeotrope with R115, and it is difficult to separate the two compounds in the azeotrope. For this reason, its has heretofore been a practice to allow most or all of the R114 to be carried off in the crude R125. However, this solution was less than satisfactory in the prior processes because it creates another problem. More particularly, R114 has a boiling point that is relatively close to that of HF and greater than the boiling point of R124. According to prior processes, the reactor effluent would first have the HCl removed by distillation and then the HCl-free stream was separated by distillation into a high boiling stream which contained the materials, such as HF, intended for recycle, and a low boiling stream which contained the desired R125. However, in order to ensure that the R114 was included predominantly in the low boiling stream and not in the recycle streams, distillation required, because of the relative volatility of R114 and R124, that substantially all of the R124 was included in the low boiling steam along with the R125. Because of the HF/R124 azeotrope, this stream would carry with it a relatively large amount of HF, which according to prior processes was removed via an undesirable caustic wash or similar operation. Therefore, the prior art solution for preventing recycle of R114 created an increased difficulty and cost in terms of the need to de-acidify additional amounts of HF. As discussed in detail below, the present inventors have discovered a process that is capable of effectively minimizing the amount of R114 recycled to the reaction while at the same time reducing the amount of HF that is contained in the crude product streams.
U.S. Pat. No. 4,843,181 is directed to a method for the production of R123 and R124 by the gas phase reaction of HF with a tetrahaloethylene or with a pentahaloethane over a chromium oxide catalyst. The process of this patent calls for the minimization of R125, and provides no teachings on handling a product stream which is rich in R125.
U.S. Pat. Nos. 4,911,792; 4,944,846; 5,094,773 and 5,560,899 all relate to methods for separating HF from R123 and R124. None of these references provide for such separations in the presence of large amounts of R125.